The present invention relates generally to variable frequency power supplies, and more particularly to, the use of a variable frequency power supply in an induction heater where the load is interruptible from the power supply.
Variable frequency power supplies are designed to operate at or near the resonance of the load that the power supply is connected to. Solid-state inverters, by their nature, have an inherent maximum and minimum operating frequency. Most such solid-state inverters cannot run without a resonant load and an output current to that load. Such inverters will automatically shut down if the current falls below a minimum current and/or the load resonant frequency falls below a minimum value or exceeds a maximum value. This is especially problematic in induction heaters since if the power supply shuts down during a heating process, the process will likely be ruined and the part being heated will likely need to be discarded. Further, much time is lost in restarting the power supply and the heating process.
In most induction heating applications, an inverter operating frequency is adequately controlled with voltage and current phase feedback to a control that provides frequency control signals to the inverter. In most of these induction heaters, the inductive heating coil is positively, or permanently, connected to the power supply such that only a defect or major breakdown will cause an interruption thereby shutting down the power supply. However, in some applications, the connection between the load and the power supply is not direct. In these applications, the load may move with respect to the power supply and the electrical connection between the load and the power supply is by a friction connector. Such friction connectors are known to arc and allow intermittent interruption, thereby changing the resonance frequency of the circuit and/or even allowing the inductance to go to infinity, resulting in the frequency being reduced below the minimum frequency of the inverter thereby shutting down the power supply. Solid State inverters are especially susceptible to shut down if the resonance frequency falls below the minimum frequency for even an instant. Therefore, solid-state inverters have generally not been used with friction connectors or with any intermittently interruptible load.
It would be desirable to be able to use a solid-state inverter power supply with a load that is prone to intermittent disconnection, or arcing, but does not shut down in response thereto.
The present invention solves the aforementioned problems by providing an interruptible frequency control power supply and a matching circuit that allows the power supply to continue to run even when the load is disconnected from the power supply.
The present invention provides an internal load in conjunction with the power supply that is sized just large enough to maintain a run state of the power supply as determined by the low frequency limit. This internal load is connected to the power supply such that when both the internal load and the external load are connected, the power supply can run at the high end of its frequency spectrum. If the external load drops out, or is disconnected, the power supply is able to continue to run at the low end of its frequency spectrum. This provides a frequency controlled power supply that is operable with infinite inductance, or zero resonance, at the load.
In accordance with one aspect of the invention, a resonant load matching circuit is disclosed that includes a circuit input connectable to a variable frequency inverting power supply to receive power operable between a maximum frequency and a minimum frequency. The matching circuit also includes a circuit output connectable to an inductive load. The load matching section is connected between the circuit input and the circuit output to provide a resonant frequency that allows the power supply to operate at or below the maximum frequency when the load is connected to the circuit output and at or above the minimum frequency when the load is disconnected from the circuit output.
In accordance with another aspect of the invention, a load interruptible variable frequency power supply is disclosed having an inverter and phase detector to supply power to an inductive load at a resonant frequency. The inverter has a minimum and a maximum operating frequency. A feedback circuit is connected to the phase detector to adjust a phase relationship between the voltage phase and the current phase based on the resonant frequency. A matching section is connected between the inverter and the inductive load. This matching section has an effect on the resonant frequency such that when the inverter is connected to the matching section and the load, the resonant frequency is at or below the maximum operating frequency. When the inverter is connected to the matching section and disconnected from the load, the resonant frequency is at or above the minimum operating frequency.
In accordance with another aspect of the invention, a method of providing power from a solid-state inverter to a load that is susceptible to disconnection from the solid-state inverter without shutting down the solid-state inverter during disconnection includes determining a minimum and maximum operating frequency of the solid-state inverter and determining an inductance of a load connectable to the solid-state inverter and a resulting operating resonance. The method also includes selecting an LC matching section connectable between the solid-state inverter and the load that permits the solid-state inverter to run at or under its maximum operating frequency when the load is connected to the solid-state inverter and to run at or above its minimum operating frequency when the load is at least partially disconnected from the solid-state inverter.